Regex escape [ character and change text result into integer

Hi.  I am using array_dims to give me dimensions of  multidimensional
array ie:

[1:5][1:2]

In my function I want to retreive the value of the second number from
array_dims (5 in example above)

This is what I am trying:

count_str = substr(dimensions from '\[\d\:(\d+)\]\[\d\:\d\]')
(this should give me 5 in the above example)

I have two problems:

1) I can't seem to escape the [ character - so how do I do this?
2) How do I turn my result which is originally text into an integer?

Regards,
David